(647b) Microfluidic Analysis of Printed Battery Electrodes

With advancement in printed electronics and integrated circuit industry the next generation of sensors and RFID tags will be physically flexible and less than sub-cm2 in foot-print. This presents a unique challenge to provide to energy storage which is self contained and high enough energy density to supply power during the desired lifetime of the sensor. With add-on printing technique one can print active material layer by layer on senors which act as electrode and electrolyte for the battery. Printing techniques are much faster than time consuming lithographic techniques which involves multiple process including a series of deposition of materials, lithographic masking and etch steps. Batteries experiences fatigue during charge and discharge cycling and the strength of the electrode plays a important role to ensure low loss of active area with battery usage. With batteries printed on flexible sensors the electrodes will experience a shear stress which will further affect the performance of the battery. The processing of the active material, printing technique and the post-printing processes play a important role in the performance of printed batteries. Understanding the effect of each of the stages on the printed battery performance plays a important role in building future generation of printed batteries which have large cycle life and low failure rate.

We use nano particle silver ink as a electrode in an alkaline battery. This ink is intended as bussing material for printed electronic circuits. In ink form, the silver nanoparticles are modified with organic additives, and upon curing a solid silver layer is formed, which has all the electrochemical propertied of bulk silver. The silver ink is printed using a dispenser printer in the form of two parallel electrodes which are 1 mm apart and less than a micron in thickness. A printed silver line was used as a reference electrode which was 1 mm from the working electrode and shows a comparable performance with respect to Ag|AgCl reference electrode. In the first charge cycle in ZnO-KOH electrolyte the negative silver electrode acts as a substrate for zinc plating and the positive electrode is oxidized from Ag to Ag2O to AgO. The open circuit potential of this cell is 1.85 V. We examine the electrochemical and the mechanical performance of the electrode which respect to different printing ,processing and operating conditions using a microfluidic device. The silver nanoparticle ink is composed to silver particle with size range of 5-10 nanometers in tetradecane solvent . The tetradecane solvent is used to prevent the agglomeration of the silver nanoparticle. The viscosity of the silver ink is 5 Cps. The silver nanoparticle ink is printed on a pre-treated glass slide using a dispenser printer where the printer tip is in contact with the substrate . The printer stage is complete programmable with a repeatable accuracy of 0.1 microns. The silver ink is printed with a dispenser which has a accuracy of 0.1 psi. The conductivity of the printed silver is a function of the baking time and temperature A microfluidic setup with cross-flow of electrolyte was used. The channel height was 300 microns and the width of the channel was 1.25 mm. The flow rate of the electrolyte can be controlled between 0.1 ml/hr to 100 ml/hr. The electrode was observed in-situ using a microscope.The flowing electrolyte removes any gas generated during the charging process at high current density, clearing the working electrode surface. The cross flow of electrolyte on the electrode causes a shear on the electrode. One can control this shear stress by controlling the flow rate of the electrolyte. This shear simulates the shear stress experienced by the electrodes during bending of flexible battery. The oxygen generated on the positive electrode during charging and the cross flow of the electrolyte delaminated the printed electrode. Experiments were carried out to separate the effect of gas generation and the flow on the strength of the positive electrode and the life of the battery.

Passing a current at high current density on a completely oxidized silver electrode causes high oxygen generation on the positive electrode. This oxygen generation delaminated the electrode and the current was passed until the electrode completely delaminated. The amount of oxygen required to completely delaminate the cell can be quantified in terms of charge passed. Experiments were carried out at different current densities and flow rates, revealing that the amount of oxygen required to delaminate the cell depended upon the amount of gas generated and not on the rate of gas generation. The electrode failure was independent of the flow rate because at high current density the electrode is always blanketed with a layer of oxygen gas and hence the shear experienced by the electrode at different current densities was negligible. This was verified with a numerical model using Comsol. The strength and the adhesion of the printed electrode was quantified in terms of the amount of oxygen required to completely delaminate the electrode. This amount of oxygen required to completely delaminate the cell was seen to be depended on the post printing possessing like baking time and temperature. The strength of the individual oxides of silver was studied by maintaining the electrode at different oxidation state of silver at constant potential. Experiments were carried at different electrolyte flow rate and the delamination of electrode was observed in situ with time. The delamination rate was seen to be dependent of the flow rate of the electrolyte and the post printing processing condition. The strength of AgO was seen to be higher as compared to Ag2O .